This invention relates to controlling the frequency of a laser signal, and particularly to removing frequency fluctuations from the laser signal.
It is well known to stabilize laser frequency (i.e., minimize frequency fluctuations known as frequency jitter) by electromechanically tuning the laser resonator in response to the instantaneous frequency of the laser beam. A control signal that is a function of errors between the detected laser frequency and a frequency-stable reference signal (such as the output of another laser or the center frequency of a high-Q atomic or molecular resonance) is applied to a piezoelectric transducer (i.e., a PZT) to move the mirror electromechanically and tune the laser resonator, thereby adjusting laser frequency. The PZT response is generally slow and has resonances at sharp frequencies in the audio region, thereby making it difficult to achieve the nearly flat bandwidth response desirable for frequency stabilization. However, the use of a PZT is generally satisfactory for removing the frequency jitter requiring control with a response bandwidth of tens or even hundreds of Hz.
It is also generally known that high speed laser frequency tuning can be achieved using an electro-optic crystal within the laser resonator. The refractive index of the electro-optic crystal is controlled by applying a DC voltage, such as the control signal discussed above, to the crystal, thereby causing the laser frequency to change and achieving frequency stabilization.
Some applications, such as the use of the laser as an optical clock in, e.g., aircraft, subject the resonator to acoustics, microphonics, and shock forces from aircraft maneuvers which cause considerable frequency fluctuations in the laser frequency. In a CO.sub.2 laser, for example, the frequency fluctuations can have swings as high as hundreds of KHz or higher and occur at fluctuation rates as high as several KHz or higher (i.e., the fluctuations can have a time scale on the order of milliseconds or less). Because PZT stabilization is generally suitable to compensate slower fluctuations, complex laser shock mounting and mechanical isolation arrangements are typically needed to aid in laser irequency stabilization.
Another known scheme for removing high-frequency jitter in tunable CW (continuous wave) lasers (such as dye lasers) is to generate an RF (radio frequency) sideband of the laser frequency using an acousto-optic or electro-optic modulator driven at, e.g., an RF frequency by a tunable voltage controlled oscillator (VCO). A sample of the laser beam is applied to an external high-Q Fabrey-Perot interferometer to produce a frequency error signal from a resonance of the interferometer. This error signal is used to tune the VCO, and hence the frequency of the RF sideband. But in a high vibration environment, the resonance of the external high-Q Fabrey-Perot interferometer is also subject to much the same variation as that of the laser resonator.
One known technique for generating a high-Q atomic or molecular resonance for laser frequency stabilization uses laser saturation spectroscopy. In this method, the laser beam is applied to an absorption resonance cell that contains atoms or molecules of gas at a predetermined pressure. The beam is reflected by mirrors in the absorption cells to establish a pair of oppositely-propagating, collinear waves in the gas. As the laser frequency is tuned across the Doppler profile of the atomic or molecular spectral line, high-Q resonance is established at the center of the Doppler profile which has a linewidth (in frequency) sizably narrower than the Doppler profile. This linewidth is a characteristic of the molecules (or atoms) which make up the gas and is a function of such factors as pressure effect and power broadening, but it is free from Doppler broadening.
The resonance of the cell defines a reference for the laser frequency and is used to extract a correction (i.e., error) signal which is applied to the laser frequency-tuning element to achieve frequency stabilization. To compensate for frequency fluctuations and thus achieve stabilization, the resonance linewidth should be narrow to yield a highly accurate and well defined frequency reference. However, high-speed frequency fluctuations and transients present under a vibration environment cannot be eliminated if they occur in a time scale comparable to the inverse of the narrow resonance linewidth. Thus, a narrow linewidth, desirable for high frequency definition and accuracy, limits the maximum jitter frequency rate that can be corrected when the laser is operated in a high vibration environment in which high speed fluctuations occur. As a result, while this frequency stabilization scheme is suitable for quiet laboratory environments, when the laser is used as a reference optical clock in an environment where excessive vibration exists, it is necessary to provide hard-to-achieve mechanical isolation from vibration (shock mounting) which adds to the size and weight of the optical clock system and is thus undesirable.
Lasers which are frequency-stabilized by the above-discussed laser saturation spectroscopy technique are typically frequency-dithered (i.e., periodically scanned in frequency) at a predetermined rate to extract the correction signal from the absorption resonance cell. Although the output of the clock will be stable, it will have an FM dither at the dither frequency rare. To enable the production of a frequency-stabilized non-dithered beam, the output beam of a second laser (which is not dithered) is typically stabilized against the dithered beam of the first laser. The output of the second laser is used as the non-dithered stable frequency clock.
One conventional method for extracting the correction signal in systems where the frequency of the laser beam is dithered uses a synchronous demodulator (i.e., a so-called "lock-in amplifier", also known as a phase sensitive amplifier) synchronized at the dither frequency to demodulate the output (i.e., the absorption response) of the absorption cell. In other known schemes in which the dithered laser frequency is scanned across the reference resonance of the cell, the absorption response of the reference resonance is digitally sampled to sense the center frequency of the resonance (that is, the frequency at which maximum absorption occurs). The samples are used to develop the correction signal, which is then applied to the laser tuning mechanism to achieve frequency stabilization.
It is also known to modify the absorption response of a molecular resonance by applying a DC electric field to the molecules in the absorption cell; this is known as the Stark effect. Such a technique has been used as an alternative to dithering the frequency of the laser beam to extract the correction signal for stabilizing the laser frequency. In this method, the DC electric field is amplitude modulated at the dither frequency, thereby modulating the absorption response at the dither frequency. The correction signal is extracted from the modulated absorption response with a synchronous demodulator synchronized to the dither frequency.